I suspect that's because it's cheaper to do blockwork and timber than cavity brickwork.
I'd be surprised if it's an "appearance" issue because I don't think there's a mechanism to insist on that (and who would think it looked right anyway?)
If it was to improve the view FROM the henge while killing the view OF the henge I'd be more than surprised I'd be ruddy outraged. Avebury's the bleeding World Heritage Site not bleedin' Wiltshire.....
Well 'clapboard' is what we used to call, and still do if you're in the trade, a modern-day weatherboard. Original weatherboard was cut directly from trees into wide boards of even thickness (mainly elm) but in more modern-times it became a tapered or 'feather-edge' board used maily for fencing panels and sheds which overlap. In the 'old' days it was used to face barns and outbuildings with, so gained the reputation of having a 'country' and rustic look to it. The old wide elm board had a natural wavy edge created by the exterior bark of the tree but in the main the boards cut today are plain-edged. I guess this is why clapboard was used on these buildings to give a more 'traditional' look, which would be some sort of a joke because the modern board has no 'age' to it at all. Another architectural ploy.
It should be called 'crapboard' cos that's what it is compared to what we used to use.